Nitrogen oxides (NOx) in the earth's atmosphere are primarily emitted by automobiles and industrial plants. Studies have shown that nitrogen oxides can be hazardous to human health and the environment when present in the atmosphere in sufficiently high concentrations. For instance, nitrogen oxides above 0.05 ppm can have hazardous effects on people in good health for an exposure of over 24 hours. See, Fritz, A. and Pitchon, V., “The current state of research on automotive lean NOx catalysts,” Applied Catalysis B: Environmental, Vol. 13 (1997), pp. 2. In particular, nitrogen oxides have been found to provoke lung infection and respiratory allergies. Nitrogen oxides have also been found to play an influential role in the formation of acid rain, smog, and general atmospheric visibility degradation. Because of the potential detrimental effects of nitrogen oxides on human health and the environment, the government has imposed several stringent regulations on NOx emissions. These lightened regulations continue to drive NOx abatement technology.
Several technologies have been developed in order to decrease NOx emissions, including passive methods using catalysts and active approaches such as electrochemical catalysis, photocatalytical approaches, plasma, laser, and so forth. Rich-burning engines, such as those used in today's gasoline-powered automobiles, use a three-way catalyst to reduce NOx emissions. The three-way catalyst typically consists of a combination of noble metals deposited on a stabilized alumina carrier. The three-way catalyst is efficient because it works together with a feedback system that directs the engine to blend air and fuel in stoichiometric proportions. By controlling the air-to-fuel ratio, the engine makes hydrocarbons and carbon monoxide available in just the right amounts to reduce NOx and unburned hydrocarbons to products like carbon dioxide, water, hydrogen gas, and nitrogen gas that are generally harmless to health and the environment. The NOx performance of the three-way catalyst, however, rapidly deteriorates in the presence of oxygen.
Removal of NOx in lean-burn engines, i.e., engines such as diesel engines that burn fuel in excess oxygen, continues to pose a great scientific challenge. There is sufficient motivation, however, to continue to pursue a practical solution for reducing NOx emissions in lean-burn engines because lean-burn engines provide better fuel economy than rich-burning engines. As previously mentioned, the NOx reducing performance of the three-way catalyst, which is the standard NOx abatement technology for gasoline engines, deteriorates rapidly in the presence of oxygen. Thus, this technology is ineffective in controlling NOx emissions in lean exhaust gases. Some of the approaches that have been considered in controlling NOx emissions in lean exhaust gases include catalytic decomposition of nitrogen monoxide (NO), selective catalytic reduction (SCR) with nitrogen containing compounds, and selective catalytic reduction with hydrocarbons (HC-SCR).
The decomposition of nitrogen monoxide to elements is described by the following equation:NO→½N2+½O2  (1)The decomposition of nitrogen monoxide is thermodynamically favored under pressures and temperatures found in diesel exhaust. However, the decomposition reaction is inhibited by a high activation energy. Therefore, a catalyst is necessary to lower this activation energy in order to facilitate this decomposition. Various catalysts have been used to decompose NO, including precious metals, metallic oxides, and zeolites-based catalysts. One of the best catalysts recently suggested for NO decomposition is copper ion-exchanged zeolite ZSM5 (Cu/ZSM5). However, the catalytic activity of Cu/ZSM5 is greatly diminished in the presence of oxygen gas and sulfur dioxide, and the decomposition of NO is effective only at low space velocities.
In SCR with nitrogen containing compounds, a nitrogen compound, e.g., ammonia or urea, is used as a reducing agent for nitrogen oxides to produce innocuous products. In reactions (2) and (3) below, ammonia is used as the reducing agent for nitrogen oxides to produce nitrogen and water:4NO+4NH3→4N2+6H2O  (2)6NO2+8NH3→7N2+12H2O  (3)The reactions (2) and (3) are favored in the presence of oxygen. In the presence of oxygen, a catalyst such as vanadium pentoxide (V2O5) supported on oxides such as TiO2, Al2O3, and SiO2 is used to facilitate the reaction. In NH3-SCR, an external source of ammonia is needed to reduce NOx to N2. NH3-SCR is widely used as a pollution reduction technique in stationary plants such as electric power plants. The toxicity and handling problems associated with ammonia, however, has limited the use of the technology in motor vehicles.
U.S. Pat. No. 5,863,508 issued to Lachman et al. describes a multi-stage catalytic reactor system which allows ammonia to be synthesized onboard a vehicle and then used to reduce NOx to N2 as previously described in reactions (2) and (3) above. The reactor system includes two units, each of which includes multiple open-ended cells. A portion of the cells in the first unit include a first stage catalyst, which is a noble metal on a support. The noble metal cannot be rhodium. Exhaust gases from combustion are passed through the first unit so that a portion of the NOx in the exhaust gases is reduced to ammonia by the first stage catalyst. The modified exhaust gas mixture is then passed to the second unit, wherein the ammonia in the modified gas mixture is reacted with the remaining NOx to yield a converted gas mixture. An external source of ammonia is not needed because the ammonia is generated in the first unit. The passage of the exhaust gases through the first and second units results in conversion of NOx, CO, and hydrocarbons to innocuous products. This technology is effective for lean burn engines.
HC-SCR was discovered during the survey of the effect of co-existing gases on the catalytic activity of Cu/ZSM5. In HC-SCR, hydrocarbons, e.g., ethane, propane, and propene, selectively react with NOx to produce nitrogen, carbon dioxide, and water:{HC}+NOx→N2+CO2+H2O  (4)There are three principal types of catalysts active for the HC-SCR, including zeolites, oxide-type catalysts, and supported noble metals. See, for example, Iwamoto, M. and Mizuno, N., “NOx emission control in oxygen-rich exhaust through selective catalytic reduction by hydrocarbon,” Journal of Automobile Engineering (1993), pp. 23-33, and Fritz, A. and Pitchon, V., “The current state of research on automotive lean NOx catalysts,” supra, pp. 10-25, for additional discussions on catalysts for HC-SCR. In this technology, additional HC must be supplied and a system is required to deliver the HC. Controlling the amount of HC needed is a great challenge. Furthermore, excess oxygen may affect NOx conversion.